Printed circuit boards (PCBs) typically comprise many layers, including metal wiring layers and organic and/or ceramic insulating layers. PCBs may also contain “optical wiring layers” comprising single-mode, multi-mode or other dimension optical waveguides. A large number of waveguides may be placed in a given area, e.g., greater than 100 waveguides per centimeter. This is important because with equipment such as servers, supercomputers and telecom switch-routers, which consist of multiple racks, each rack having an aggregate data rate reaching multiple terabits per second (Tb/s), i.e., potentially on the order of tens of Tb/s, a larger capacity is needed.
Further, the waveguides are easily patterned, routed and are insensitive to electrical interferences within the PCB. The use of such waveguides provides several notable benefits, including the ability to transmit data in the optical domain.
The waveguides may be integrated in the PCB. By integrating the waveguides within the PCB, several advantages may be achieved. For example, the individual waveguides are protected from the environment, can be routed directly underneath opto-electronic (OE) modules and allow valuable top PCB surface space to be preserved for other components.
A waveguide typically comprises a cladding layer and a core formed in the cladding layer. Such a waveguide may be fabricated by first forming a cladding layer on a substrate. On top of the cladding layer, the core layer is next deposited and patterned to achieve lateral definition. The core layer is covered with another cladding layer, to bury the core layer.
The refractive index of the core is selected to be larger than either of the cladding layers. The materials making up the cladding layers and the core layer are optically transparent, to obtain low propagation loss. Multi-mode waveguides typically have a cross-sectional geometry of about 50 square micrometers (μm).
OE modules may contain optical transceivers, for example, vertical cavity surface emitting lasers (VCSELs) and photodiodes (PDs) which serve to transmit and receive optical signals, respectively. These OE modules can reside on/in the PCB, adjacent to/integrated with processors, application specific integrated circuits (ASICs) and memory controllers, whenever dense, high speed optical interconnects are required.
Precise alignment of an optical component (e.g., with an accuracy of about five μm), which can be the OE module itself or another optical component, such as a lens or a mirror, is needed to couple the light, e.g., from the VCSELs into the waveguides and/or from the waveguides to the PDs. Various typical coupling concepts exist. These coupling concepts, however, require first positioning the optical components in a rough proximity to the waveguides (e.g., with an accuracy of greater than or equal to about 50 μm) and then further actively aligning the optical components with the waveguide core, to attain the precise alignment accuracy needed. These “active alignment” steps are however inaccurate, as well as, time consuming and cumbersome for the operator.
Therefore, techniques are needed for aligning optical components with waveguides without active alignment steps.